During growth and division of cells in the body, the DNA at the ends of chromosomes becomes shorter owing to an inability of cells to fully replicate these sites, which are called telomeres. Excessively short telomeres are dysfunctional and cause cells to stop dividing, and in many cases commit suicide. Thus, telomere shortening provides a timer that limits the growth potential of most cells in the body. How cells escape this limit is a critical question for understanding the genesis of cancer. The work proposed here uses the fruitfly, Drosophila melanogaster, as a model system to identify the genetic mechanisms either guide cells to suicide, or allow them to escape that fate. Specially engineered chromosomes are used to provide a system that efficiently, and on command, causes loss of one telomere in a cell. Most cells die in response, but a few cells escape and divide repeatedly. The experiments described in this proposal will define the mechanisms that cells use to escape death, providing important insight into the earliest stages of carcinogenesis. In reproductive cells, the loss of a telomere is often followed by construction of a new telomere. The proposed work will define the genetic control over this chromosome healing. Inappropriately healed chromosomes are associated with a number of human disorders. Moreover, healing is a common feature of cancer cells, adding further relevance to human health.